Reddit mentions of Superheadz 35mm Back Adapter for Holga 120 Cameras

You can load 35 mm film into a holga 120 by putting spacers, such as foam, strofoam, stacked pennies, or anything the will securely hold the 35mm film case in place of the taller 120 case. You must then tape up the back and sides of the camera to avoid light leaks, unless you want them. Always put at least one layer of tape on the back to not completely ruin your film.

You can get a 35mm back for the holga 120 if you're not comfortable with do a DIY solution, but DIY is what makes toy cameras really great. The difference in buying the popular, cheap factory-made 35mm back is the frame is cropped to fit the standard size of 35mm frame. This means more individual photos, but takes away the ability to do sprocket hole shots like this and makes the shot more narrow. If you go the DIY route, you can make your own 35mm sized frame by cutting a 35mm frame-sized square from a piece of cardboard and putting it in your 120.

There is a 35mm film back for 120 Holga's that is pretty cool, though a bit more expensive. The 35mm back made by superheadz turns the 120 into a fully featured 35mm holga camera, with a 35mm film counter, frames that allow sprocket-shots, and all the 35mm mechanics built right in. If you want to buy a 35mm back, you should buy this one.
http://www.amazon.com/Superheadz-35mm-Adapter-Holga-Cameras/dp/B001W4JOWI/ref=sr_1_2?ie=UTF8&qid=1372035953&sr=8-2&keywords=holga+35mm+back
Again, toy camera photography is all about DIY and imperfections, so I would encourage going the DIY route for the getting most exploration, invention and adventure possible with a holga 120

loading 35mm into your 120 means you have to wind the film in very specific rotations, as 120 winder doesn't click or stop after each frame. the one exception of this is using the superheadz back. See here: http://photondetector.com/tools_ref/135-advance/

Also re-winding film must be done in a dark area, like a bathroom or closet without any lights on. 120 cameras don't rewind, so you have to take the film out and rewind it by hand. The superheadz back has a winder built into it.

squarefrog (who is also a member of this subreddit) has a great guide to using 35mm in a 120 holga: http://www.squarefrog.co.uk/techniques/using-35mm-film.html

I started with the holga 135bc, which takes 35mm film and superimposes a fake vignette over every photo. The vignette effect is great, but I really wish I just started with a 120. The 135 series is very limiting compared to the 120s.

processing black and white film is easy and cheap and does not require any expensive equipment. Developing color film requires more chemicals and very accurate temperature control. Color is usually not processed outside of professional labs.
http://www.shawnhoke.com/2011/01/developing-black-and-white-film-at-home/